This invention relates to apparatus and methods for controlling the alternating current (A.C.) output voltage of an electrical power generating system including a generator having a winding the direct current (D.C.) energization of which influences the A.C. output voltage. In particular, this invention relates to apparatus and methods for controlling the energization of the winding in controlling the A.C. output voltage.
Heretofore, it is has been recognized that when an A.C. generator has a varying load, its output voltage tends to vary. In fact, when an A.C. generator rejects the load (i.e., due to an overload of such a magnitude that a protective breaker opens the circuit supplying the load), or has its electrical load suddenly reduced, the output voltage can rise abruptly. It is highly desirable to maintain the A.C. output voltage essentially at an established nominal system voltage and to control the A.C. output voltage to prevent significant departures from the nominal system voltage under any conditions.
D.C. power has been applied to a field winding of an A.C. generator through a motor controlled rheostat which provides adjustment of the generator terminal voltage. A field breaker, a disconnect switch, and field discharge resistor have been connected across the field winding of the generator. Such a system is relatively slow in its response to occurrences of load rejection. Moreover, such a system also involves continuous dissipation of power and represents lost energy and lost revenue, and increases power house ambient temperature with a consequent reduced longevity of equipment affected by the increased ambient temperature.
An A.C. generator, or alternator, typically has an armature winding for producing an A.C. output voltage and a field winding externally supplied with direct current energization which controls the A.C. output voltage of the generator. When the field is wound on the stator, the armature winding is on the rotor and the A.C. output voltage is taken from the armature by slip rings and brushes. When high A.C. voltages are required, the field winding is part of the rotor and the armature is part of the stator with the A.C. output voltage being directly taken from the stator. The alternating current can be generated at high voltages because no movable contacts are required to connect the armature to an electrical load as would be the case if the armature were revolving. The rotating field winding can be externally supplied with direct current through brushes and slip rings.
The use of brushes is avoided altogether by having the rotating field winding of the alternator supplied with electricity by a generator, known as a rotary exciter, mounted on the same shaft as the alternator rotor. The rotary exciter, in turn, has a field winding of its own which is wound on its stator and is directly energized by D.C. The rotating alternator field winding is then directly supplied with D.C. from the exciter armature, e.g., through rectifying diodes. Both the rotating field winding of the alternator and the field winding of the rotary exciter are herein regarded as further examples of a winding, the D.C. energization of which controls the A.C. output voltage of the generator.
A unit known as a regulator provides the D.C. energization. One type of regulator is called a shunt static exciter (SSE), which is a unit widely used in the industry and readily available. Such regulators exhibit many useful features, but the speed with which they can reduce the generator excitation is limited by the inherent time constant of the winding, the D.C. energization of which controls the A.C. output voltage.
Some known regulating systems are disclosed in the following coassigned patents U.S. Pat. No. 3,518,528 "Generator Voltage Regulator with Reactor Sensing Means," U.S. Pat. No. 3,316,479 "Regulating Systems for Alternating Current Generators," and U.S. Pat. No. 4,264,856 "System for Maintaining Excitation of an Alternating Current Generator During Excessive Output Current Conditions."